1. Field of the Invention
The present invention relates to an electrophotographic printing apparatus comprising, in the inside of its housing, developing means, image transfer means and thermally fixing means.
2. Description of the Prior Art
These years, with the progress of the office automation for business equipment and machines, the demand of printers as terminal equipment of computers has been more and more increased. Inter alia, electrophotographic printing apparatuses such as laser printers which carry out nonimpact type printing have been rapidly spread in use because of low noise and a high printing quality.
Heretofore, such an electrophotographic printing apparatus uses a ventilating fan for forced ventilation of air outside of its housing in order to protect the apparatus against gas such as ozone or NOx produced through corona discharge by a developing means and an image transfer means so as to ensure the printing quality, and in order to radiate remaining heat from a thermally fixing means.
Explanation will be hereinbelow made of a conventional electrophotographic printing apparatus. Referring to FIG. 4 which is a sectional view illustrating an essential part of the conventional electrophotographic printing apparatus, a photosensitive medium drum 1 is electrified through corona discharge by a charger 2, and is then formed thereon with an electrostatic latent image by exposure light from an exposure unit 3. Thereafter, the electrostatic latent image is developed into a visual image with the use of toner by a developing unit 4. The visualized toner image is transferred, by an image transfer unit 5, onto a printing sheet 6 which has been fed from a paper feed part (which is not shown) in synchronization with the rotation of the photosensitive medium drum 1.
The printing sheet 6 on which the toner visualized image has been transferred is fed to a fixing part composed of a heating roller 7 and a pressing roller 8, and after fixing of the toner visualized image, the sheet 6 is discharged from a paper discharge part 16. Meanwhile, toner remaining on the outer surface of the photosensitive medium drum 1 after the image-transfer is removed therefrom by means of a cleaning blade 9, and then the photosensitive image 1 is deelectrified by a discharger 10 so as to have a uniform surface potential.
With the repetitions of the above-mentioned developing steps, several images are formed. The photosensitive medium drum 1, the charger 2, the image-transfer 5, and the fixing part composed of the heating roller 7 and the pressing roller 8, as mentioned above, are arranged, in general, horizontally within a housing 11.
Next explanation will be made of a ventilation system for the thus arranged electrophotographic printing apparatus. During the above-mentioned image formation, the charger 2 and the image transfer 5 using corona discharge produce gas such as ozone or NOx. Should the produced gas such as ozone or NOx be accumulated in the housing 11, the use lives of several components in the housing 11 and the printing quality would be detrimentally affected. Further, remaining and extra heat from the fixing part heats up several parts therearound, and accordingly, the use lives of the these parts and the printing quality are also detrimentally affected.
Accordingly, a ventilating passage 12 is formed communicating gas and heat sources to the outside of the housing, and a ventilating fan 14 for driving out the gas or the like is provided in the vicinity of a vent port. Thus, air streams A, B, C as indicated by the arrows are induced so that produced gas such as ozone or NOx and extra heat generated from the heating roller 7 in the fixing part are expelled outside of the housing 11. At this time, ozone generated from the charger 2 and the image transfer 5 is sucked up by the air streams A, B, and is then absorbed by an ozone absorbing filter 15 provided just before the ventilating fan 14. Further, the extra heat from the heating roller 7 is radiated outside of the housing 11 by the air stream B, C as indicated by the arrows.
However, the above-mentioned conventional electrophotographic printing apparatus using the ventilating fan 14 has raised many problems such that the ventilating fan 14 is expensive, and the miniaturization of the apparatus is limited because an extra space should be ensured for the attachment of the ventilating fan 14, and such that the ventilating fan 14 produces noise.